The present invention relates generally to an illumination apparatus, and more particularly to an illumination apparatus and an exposure apparatus used to expose an object, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an illumination apparatus for an exposure apparatus that uses the X-ray and the extreme ultraviolet (“EUV”) light as an exposure light.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with recent demands for finer semiconductor devices, a shorter wavelength of ultraviolet light has been promoted from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing of a semiconductor device, and a reduction projection exposure apparatus that uses the EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet (which is referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
A light source of the EUV exposure apparatus includes a laser produced plasma (“LPP”) light source that irradiates the laser onto a target and captures the EUV light from the generated plasma, and a discharge plasma light source that pinches or converges the plasma generated from the discharge under a low-pressure gas atmosphere, and captures the EUV light from the pinched plasma.
The plasma is a light emitting point in these light sources, and the bigger plasma provides more EUV light. The EUV light emits from the plasma approximately isotropically and the bigger capture angle provides more EUV light.
The EUV light source in the EUV exposure apparatus needs the etendue of 1–3.3 mm2 sr or smaller. The etendue is defined by S×Ω, where S is a plasma size, and Ω is a solid angle at which the light is captured from the plasma. The upper limit of the etendue restricts a size of the light source and the capture angle.
The upper limit of the etendue is determined, because an ideal or aplanatic optical system does not change the etendue and an optical system that has the aberration has a large etendue. Since the projection optical system in the exposure apparatus is an optical system that sufficiently reduces each aberration, the etendue of the exposure light irradiated from the projection optical system onto the wafer as a substrate is approximately equal to the etendue of the light captured from the reticle as an original to the projection optical system. For example, when the light having an NA of 0.25 (or a solid angle of 0.2 sr) enters an area having a width of 2 mm and a length of 26 mm on the wafer, the etendue becomes 10.4 mm2 sr. A ratio between the NA of the light irradiated onto the reticle from the illumination optical system and the NA of the light captured from the reticle to the projection optical system is called a coherence factor σ and set to a value smaller than 1. The coherence factor σ relates to the square of the etendue: For example, when σ is 0.8, the etendue of the light irradiated from the illumination optical system to the reticle is 6.7 mm2 sr.
In general, the light emitting point has an intensity distribution and a positional offset, and an integrator is used to reduce these influences. The integrator divides the light into plural secondary light sources, combines the lights from the secondary light sources, and makes the light intensity distribution uniform. Thereby, the light irradiated onto the reticle has a uniform light intensity. In the integrator as a thus serving optical element, the exit light's etendue is much greater than the incident light's etendue. Thus, the EUV exposure apparatus is required to maintain the etendue of the light that emits from the light emitting point 1–3.3 mm2 sr or smaller.
The fundamental object of the extensive research and development of the EUV light source is to propose a light source that introduces the symmetrical light to an illumination optical system with respect to the optical axis. See, for example, Japanese Patent Application No. 2002-6096. On the other hand, a light source that uses plural X-ray sources has also been proposed. See, for example, Japanese Patent Application No. 9-115813 and 11-40480.
While the above estimation of the etendue assumes that the light irradiated onto the wafer has different width and length directions, the light use efficiency lowers in the narrow width direction when the light symmetrical with respect to the optical axis enters the illumination optical system. In this case, the non-used light heats the illumination optical system and thermally deforms the optical element, causing the deteriorated optical performance.
Accordingly, it is conceivable to improve the light use efficiency by reducing the etendue from the light source and by using the integrator for the longer width direction so as to enlarge the length or the angle and appropriately set the etendue. However, in this case, the small etendue reduces the intensity of the light captured from the light emitting point. This results in the small light intensity for irradiating the wafer, and causes the lowered throughput of the exposure apparatus.